Cracking resistance of electrical encapsulating materials based on cured epoxy compositions, for example, at low temperatures, is an important technical characteristic. Encapsulating insulation materials in electrical applications, such as epoxy insulations, surrounding metal coils in electrical instrument transformers are prone to cracking at low temperatures, for example, below 0° C. This is mainly due to the difference in the Coefficient of Thermal Expansion (CTE) of the epoxy insulation system which generally is comparatively high and the Coefficient of Thermal Expansion (CTE) of the metal coil which is comparatively low. Epoxy resins being used as encapsulant for electrical equipment do not fulfill the low temperature cracking requirements.
In addition, encapsulating materials can exhibit an accuracy class standard of sufficient flexibility in order to prevent the metal insert from bending. Accuracy class is a standard used in the case of electrical appliances such as instrument transformers, the current error of which should remain within specified limits. Accuracy class is a function of the curing shrinkage that occurs in a curing epoxy resin composition upon cooling, whereby said shrinkage occurs when the glass transition temperature (Tg) of the epoxy resin composition is above room temperature (RT). A high glass transition temperature (Tg) of the epoxy resin composition that on curing is much higher than room temperature (RT<Tg), generally causes a high curing shrinkage of the epoxy resin composition in its glassy state, which subsequently causes internal stress of the cured composition. The internal stress then results in bending of the encapsulated metal insert, such as a magnetic core in an instrument transformer, and hence making the electrical commodity go out of accuracy class. The closer the glass transition temperature of the epoxy resin composition is to room temperature, the lower is the internal stress upon curing.
For improving the cracking resistance of the insulator, U.S. Pat. No. 3,926,904 and U.S. Pat. No. 5,939,472 discloses rubber inclusions into the epoxy composition. U.S. Pat. No. 4,285,853 and U.S. Pat. No. 5,985,956 disclose the use of nanoclay such as Montmorillonite and Wallastonite along with a silica filler in epoxy compositions. Nanoclays lower the overall Coefficient of Thermal Expansion (CTE) of the cured epoxy which improves their low temperature cracking resistance. However, the major shortcoming of this technique is the difficulty in exfoliating the nanoclay particles for obtaining a sufficiently increased surface area contact and maximum CTE reduction. Inclusion of such components into the epoxy resin composition is technically difficult, generally changes the physical properties of the epoxy resin composition and is cost and processing intensive.
For improving the accuracy class standard of sufficient flexibility and lowering the glass transition temperature (Tg) of an epoxy resin composition, U.S. Pat. No. 6,322,848 discloses the addition of aliphatic ether compounds containing terminal epoxy groups such as the glycidyl ether of polypropylene glycol or the glycidyl ether of 1,6 hexanediol as well as the addition of mono glycidyl ethers, such as a glycidyl ether of a (C12-C14)-alcohol.
U.S. Pat. No. 3,878,146 suggests the addition of oxidized vegetable oils, such as oxidized linseed or soybean oil, to epoxy resin compositions in order to lower the glass transition point.
Many electrical types of equipment that require encapsulation, such as the metal coil of an instrument or distribution transformer, are manually and heavily padded with cotton tape before casting. The padding acts as cushioning and reduces the stress on the coil resulting from curing shrinkage of the cured epoxy resin composition upon cooling. However, such manual padding requiring additional production steps is time consuming, and considerably raises the production costs of the transformer.
Documents discussed above disclose additives either for improving the cracking resistance of electrical encapsulating materials made from cured epoxy compositions or for improving the accuracy class standard of sufficient flexibility of such epoxy resin compositions. These documents, however, do not disclose, for example, improving the cracking resistance and at the same time also the accuracy class standard of sufficient flexibility for electrical encapsulating materials based on cured epoxy resin compositions as may be desirable or required when these compositions are used as an encapsulant for encapsulating insulation materials in electrical applications surrounding magnetic cores such as in medium to high voltage electrical equipment, for example in electrical instrument or distribution transformers.